Strange Shock Absorber Behaviour
Discussion
I've already posted a similar question over on Scoobynet, but no responses have been forthcoming yet. As it could be construed as a general shock absorber question and not Subaru or Impreza specific, I though it worth asking you guys on here, too!
Anyway... Whilst trying to determine where the annoying intermittent 'clonk' is coming from on the back of my 1997 Impreza Turbo 2000 (Wagon), I've noticed some very strange behaviour from my shock absorbers.
Firstly, I should mention that the suspension is all original - no aftermarket bits and (as far as I'm aware). These are still the original shocks and springs - so 17 years old and 100K miles. For general everyday driving, the suspension still feels pretty good - nice and firm as it should be. I've experienced cars with worn shocks before, and this doesn't exhibit the bouncyness, wallowing, dip/dive characteristics or instability that I'd usually associate with worn shocks when driving - in fact it drives very well, considering the age of the components.
But... I've noticed that if I leave the car standing for, say 5 minutes, and then bounce the suspension at the back, I get an initial large amount of bounce for the first couple of rebounds - very similar behavior to that which I would expect for worn shocks. However, after those first few rebounds, the shocks appear to stiffen up, the suspension becomes firm (as it should be) and seems to stay that way until I then leave the car again for a another 5 minutes, whereupon I get the initial weak shock symptoms again, until after a couple of rebounds.
I've tested the front suspension in the same primitive way (i.e me bouncing it!), and it seems to do exactly the same thing - initially weak, and then firming up after a couple of rebounds.
I've never known shocks to behave like this before - in my experience they've always been consistent in rebound rate - whether it's firm, or weak. I really don't understand how they can go from 'weak' to 'firm' within a few rebound or is it simply indicative that it's time they should be replaced?
Anyway... Whilst trying to determine where the annoying intermittent 'clonk' is coming from on the back of my 1997 Impreza Turbo 2000 (Wagon), I've noticed some very strange behaviour from my shock absorbers.
Firstly, I should mention that the suspension is all original - no aftermarket bits and (as far as I'm aware). These are still the original shocks and springs - so 17 years old and 100K miles. For general everyday driving, the suspension still feels pretty good - nice and firm as it should be. I've experienced cars with worn shocks before, and this doesn't exhibit the bouncyness, wallowing, dip/dive characteristics or instability that I'd usually associate with worn shocks when driving - in fact it drives very well, considering the age of the components.
But... I've noticed that if I leave the car standing for, say 5 minutes, and then bounce the suspension at the back, I get an initial large amount of bounce for the first couple of rebounds - very similar behavior to that which I would expect for worn shocks. However, after those first few rebounds, the shocks appear to stiffen up, the suspension becomes firm (as it should be) and seems to stay that way until I then leave the car again for a another 5 minutes, whereupon I get the initial weak shock symptoms again, until after a couple of rebounds.
I've tested the front suspension in the same primitive way (i.e me bouncing it!), and it seems to do exactly the same thing - initially weak, and then firming up after a couple of rebounds.
I've never known shocks to behave like this before - in my experience they've always been consistent in rebound rate - whether it's firm, or weak. I really don't understand how they can go from 'weak' to 'firm' within a few rebound or is it simply indicative that it's time they should be replaced?
17 years, 100k and I don't expect any Scooby is driven gently it's time to change them.
As for the symptoms it could be as simple as shock fluid draining down when at rest and you pushing the shocks in compression forces fluid past the plates inside but I am clutching at straws as I don't know what spec the OEM shocks are, they may have a different ratio for compression & rebound
As for the symptoms it could be as simple as shock fluid draining down when at rest and you pushing the shocks in compression forces fluid past the plates inside but I am clutching at straws as I don't know what spec the OEM shocks are, they may have a different ratio for compression & rebound
The clonk sounds like the noise I had on mine. I had recently had them rebuilt and someone forgot to tighten the valve that holds the gas in, so it leaked out. Took them back and they regassed them while I waited.
The clonk sounded awful, I thought something was going to drop off! Apparently it is the fluid cavitating because of the loss of pressure.
At that age I think you probably need to refurb/replace.
The clonk sounded awful, I thought something was going to drop off! Apparently it is the fluid cavitating because of the loss of pressure.
At that age I think you probably need to refurb/replace.
Edited by Colin RedGriff on Wednesday 25th March 12:21
Sorry - I should have been more specific. The 'clonk' issue has been resolved. The replacement rear drop-links that I fitted 18 months ago must have been of poor quality as the rubber bushes had completely deteriorated in less than 15K miles. I remember noticing at the time of fitting that they weren't quite the same design as the ones I removed. I didn't think they'd die that quickly though! The 'new' replacements appear to be of much better quality and are the same as the original items that I had replaced first time around. I've also just realised that my earlier comment about the suspension being completely original, wasn't quite accurate based on this, either!
The question regarding the shocks was one more of curiosity, than anything else as it's not something I've come across before. I honestly wouldn't have even noticed that they behaved this way, unless I was investigating the 'clonk'. As the car still seems to handle perfectly, I'm really not keen to replace something that still seems to be working fine for regular driving on the basis that it's [X] years old, or has covered [X] number of miles, especially at the cost of £250+ for shocks all round (I'd want to replace both front and back, if doing them), plus new top-mounts and probably springs too, whilst I'm there - so estimating around £600 just for parts. If it was bouncing all over the place, then I wouldn't hesitate!
-EDIT
I should have added here, that I've ended up completely replacing suspension on several cars that I've owned previously, due to feeling that it was a bit soft and could be harder - but in this case, the suspension still feels very good when driving. I don't want to come across as someone who isn't prepared to pay out to maintain and replace bits!
-END EDIT
In this particular case (and I realise I'm probably an exception to most here), the car's previous owner was a family member who'd owned it since it was 18 months old, so I know it's history well. Whilst I'm certain there have been occasions where it wasn't driven gently (in both my ownership and his!), I don't understand why the suspension should have suffered any more than any other car just because it's an Impreza?
The theory about the oil draining down and then being pushed back around when compressed makes sense in my mind. I know that later 'Classic' Imprezas use inverted struts which are very prone to knocking issues (which may be what Colin had), but the earlier UK Impreza just used normal shocks - the same as were used on the Legacy, I think - softer, for our pot-holed roads than the JDM imports.
The question regarding the shocks was one more of curiosity, than anything else as it's not something I've come across before. I honestly wouldn't have even noticed that they behaved this way, unless I was investigating the 'clonk'. As the car still seems to handle perfectly, I'm really not keen to replace something that still seems to be working fine for regular driving on the basis that it's [X] years old, or has covered [X] number of miles, especially at the cost of £250+ for shocks all round (I'd want to replace both front and back, if doing them), plus new top-mounts and probably springs too, whilst I'm there - so estimating around £600 just for parts. If it was bouncing all over the place, then I wouldn't hesitate!
-EDIT
I should have added here, that I've ended up completely replacing suspension on several cars that I've owned previously, due to feeling that it was a bit soft and could be harder - but in this case, the suspension still feels very good when driving. I don't want to come across as someone who isn't prepared to pay out to maintain and replace bits!
-END EDIT
In this particular case (and I realise I'm probably an exception to most here), the car's previous owner was a family member who'd owned it since it was 18 months old, so I know it's history well. Whilst I'm certain there have been occasions where it wasn't driven gently (in both my ownership and his!), I don't understand why the suspension should have suffered any more than any other car just because it's an Impreza?
The theory about the oil draining down and then being pushed back around when compressed makes sense in my mind. I know that later 'Classic' Imprezas use inverted struts which are very prone to knocking issues (which may be what Colin had), but the earlier UK Impreza just used normal shocks - the same as were used on the Legacy, I think - softer, for our pot-holed roads than the JDM imports.
Edited by benharris on Wednesday 25th March 20:09
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